Flash memory, FeRAM, MRAM, phase-change memory, and the like are conventionally known types of non-volatile memory. Recent years have brought about a demand for high-density memories for use in personal digital assistants and the like, and therefore non-volatile memories that employ a phase-change technique are attracting widespread attention and various modifications to them have been proposed (WO, A1 No. 98/19350 (Japanese Unexamined Patent Publication No. 2001-502848), etc.).
For example, Japanese Unexamined Patent Publication No. 1997-282723 discloses an information-recording device in which the writing or deleting of data is performed by bringing an electrically conductive probe into contact with the surface of a recording medium that contains an amorphous semiconductor thin film.
WO, A1 No. 98/336446 (Japanese Unexamined Patent Publication No. 2001-504279) discloses a phase-change non-volatile memory in which, as shown in FIG. 10, a phase-change material layer 83 is formed between a lower electrode 81 and an upper electrode 82, whereby the phase-change material layer 83 can be charged through the lower electrode 81 and the upper electrode 82. The phase-change material layer 83 comprises a chalcogenide material whose phase is reversibly changeable between an amorphous (non-crystalline) state of high resistance and a crystalline state of low resistance. The material is changed to an amorphous or crystalline state by the application of current, thereby controlling its resistance value. For example, when storing (writing) data, the phase-change material layer 83 is changed from the amorphous state to the crystalline state, lowering the resistance value. When deleting data, the phase-change material layer 83 is changed from the crystalline state to the amorphous state, raising the resistance value. The difference in resistance value is thus read to allow the phase-change material layer 83 to serve as a memory.
In the structure shown in FIG. 10, a joint portion 81a disposed between the lower electrode 81 and the phase-change material layer 83 is isolated by an insulating layer 84. Here, silicon oxide is preferable as the material for the insulating layer 84. However, when the joint portion 81a is insulated using silicon oxide, which is an inorganic dielectric having a relatively high thermal conductivity, a large amount of electric power is needed to write or delete data. This makes it difficult to save power.
On the other hand, when the insulating layer 84 is made of an organic dielectric, this structure not only writes and deletes data with little power consumption but also makes the insulating layer 84 cheaper, lighter in weight, and capable of coping with bending deformation.
However, when using only an organic dielectric as the material for the insulating layer 84, the number of data rewriting cycles is insufficient because the heat withstand temperature of the organic dielectric is lower than the melting point of the phase-change material. In other words, while deleting data, the joint portion 81a that is disposed between the lower electrode 81 and the phase-change material layer 83 momentarily generates heat, which varies the temperature of the phase-change material layer 83 above its melting point (for example, 600° C. or higher). On the contrary, when polyimide, which exhibits excellent heat resistance properties compared to other organic dielectrics, is used as the material for the insulating layer 84, the insulating layer 84 can only withstand temperatures of approximately 500° C., even if the heating is momentary. As a result, the insulating layer 84 near the joint portion 81a decomposes while repeatedly rewriting data. This deteriorates the electrical properties and mechanical stability of the lower electrode 81 and the phase-change material layer 83.